The present invention relates to chemical sensors, particularly a microbend fiber-optic chemical sensor adapted to detect certain chemical species.
Chemical detection is a necessity for many applications. Most chemical sensors currently used are electrical. While performing adequately in many applications, they are susceptible to electromagnetic interference and can cause unacceptable sparking in potentially explosive environments.
The development of fiber-optic technology and the application of the fiber-optic technology to chemical sensors has permitted construction of chemical sensors with at least two advantages over their electrical counterparts: immunity to electromagnetic interference and safe performance in a potentially explosive environment. Furthermore, the geometric flexibility of optical fibers and their ability to confine light allow for remote measurement in chemically aggressive environments.
However, existing fiber-optic chemical sensors require modification of at least one constituent, and frequently involve removing an original cladding over certain sections of the fiber and replacing it with cladding that reacts with the chemical compositions being monitored. This reaction can produce an absorptive cladding and thus attenuated total internal reflection, or a change in a cladding reflective index and thus reduced guidance, particularly if the light is sent into the optic fiber off-axis. The reaction can also modify the cladding so as to enhance or suppress fluorescence generated within it when xe2x80x9cpumpxe2x80x9d light is sent through the fiber.
Known fiber-optic chemical sensors can also have sensitivity problems, either because the reaction does not strongly modify the cladding or because only a small fraction of the fluorescence is coupled into the guided modes of the fiber. In addition, these modifications require careful post-processing of the fiber.
There is a current need for a fiber-optic chemical sensor that will not require modification of its constituents or alteration of the cladding and that avoids the sensitivity problems of known fiber-optic chemical sensors.
U.S. Pat. No. 5,132,539, dated Jul. 21, 1992, to Jonathan E. Weiss, discloses a microbend fiber-optic strain sensor that relies on permanent microbends, impressed in a 254-micron plastic optical fiber, to induce optical scattering out of the fiber core. The patented strain sensor relied on the fact that tension in the fiber caused xe2x80x9cout-scatteringxe2x80x9d to rapidly diminish through the unfolding of the microbends.
The present invention solves the problems typical of known fiber-optic chemical sensors. According to the present invention, there is provided a microbend fiber-optic chemical sensor for detecting certain predetermined chemicals in a chemical-bearing liquid or gas, by detecting a change in the characteristics of fluorescent light conducted through optical fibers, and a method for its use. Unlike known fiber-optic chemical sensors, no removal of the cladding and re-coating of the fiber are necessary in the present invention.
Unlike the previously described, patented strain sensor, the present invention relies on both xe2x80x9cout-scatteringxe2x80x9d and xe2x80x9cin-scatteringxe2x80x9d, and does not rely on strain. In the present invention, all fibers are relaxed so that no thermally-induced changes in tension occur.
The microbend fiber-optic chemical sensor of the present invention comprises at least one optical fiber, having a microbend section (a section comprising small undulations in the axis of the fiber(s)), for transmitting and receiving light. In the transmission phase, pump light is sent through the microbend section of the optical fiber(s), where the microbends cause optical scattering of the pump light out of the fiber core. In the receiving phase, changed fluorescent radiation resulting from the interaction of the light with the chemical, either directly or indirectly, is scattered into the microbend section of the optical fiber(s) and is then conducted by the fiber(s) to an optical detector.
In one embodiment, the analyte is xe2x80x9cpumpedxe2x80x9d directly with light that is out-scattered from the optical fiber(s) in the transmitting phase. The out-scattered light excites fluorescence radiation in the chemical species to be detected, and the fluorescence radiation characteristic of the particular molecular species xe2x80x9cin-scattersxe2x80x9d into the fiber(s), in the receiving phase, and travels to a detector where the intensity of the light, indicative of the concentration of the species, is detected.
In alternative embodiments, the microbend fiber-optic chemical sensor of the present invention comprises a fluorescer-bearing membrane in interactive contact with the transmitting and receiving optical fiber(s). The fluorescer-bearing membrane is structurally formed either as a separate structure or a coating of the fiber(s) of the sensor, in either case remaining in interactive contact with the optical fiber(s) of the sensor.
Where the microbend fiber-optic chemical sensor comprises a fluorescer-bearing membrane, the fluorescent material of the membrane interacts with the specific chemical or chemicals to be detected. Upon contact with the fluorescer-bearing membrane, the pump light scattered out of the fiber(s) induces fluorescence radiation, and the interaction between the fluorescers of the membrane and the chemical changes various fluorescence characteristics of the fluorescence radiation induced by the contact. The changed fluorescence radiation is then in-scattered to the optical fiber(s) in the receiving phase, and travels to a detector where the nature of the changes determine the presence and other characteristics of the chemical.
The exact nature of the change in fluorescence radiation depends on the chemical to be identified. The fluorescence radiation is enhanced or quenched, depending on the chemical in the sample, causing increase or reduction, respectively, in its optical power and, therefore, in the signal that goes to the detector. When the changed optical power is detected, the presence (and other characteristics) of the specific chemical can be determined. The change in the fluorescence radiation may also be reflected by a shift or change in shape in its fluorescence spectrum, in which case spectral analysis is used to detect the chemical.
Additional objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying figures, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.